Process for producing acrylic elastomer

ABSTRACT

An acrylic elastomer capable of giving a vulcanization product having a distinguished tensile strength and a good stretching even by cross-linking with an organic peroxide can be obtained by using an organic peroxide having a non-conjugated double bond as a polymerization initiator in polymerization of at least one of (a) alkyl acrylates having an alkyl group of 1 to 8 carbon atoms and (b) alkoxyalkyl acrylates having an alkoxylalkyl group of 2 to 8 carbon atoms.

This is a division of application Ser. No. 07/728,892, filed Jul. 11,1991, now U.S. Pat. No. 5,194,540, which is a continuation of07/489,975, filed Mar. 7, 1990, now abandoned and the benefits of 35 USC120 are claimed relative to it.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for producing an acrylic elastomer,and more particularly to a process for producing an acrylic elastomercross-linkable with an organic peroxide.

2. Description of the Prior Art

Acrylic elastomers comprising polymers of acrylic acid esters areusually subjected to copolymerization with polyfunctional monomersserving as cross-linking points because the acrylic elastomers have nodouble bonds in the main chain. When the acrylic elastomers arecopolymerized with polyfunctional monomers capable of introducing doublebonds into the main chains of the resulting copolymers and when theresulting copolymers are cross-linked with an organic peroxide, theresulting vulcanization product is inferior in the tensile strength andstretching than other vulcanization products and thus needs furtherimprovements in these respects.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for producingan acrylic elastomer capable of giving a vulcanization product having adistinguished tensile strength and a good stretching even ifcross-linked with an organic peroxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Production of acrylic elastomer that can attain the object of thepresent invention is carried out by using an organic peroxide having anon-conjugated double bond as a polymerization initiator in thepolymerization of at least one of (a) alkyl acryaltes having an alkylgroup of 1 to 8 carbon atoms and (b) alkoxyalkyl acrylates having analkoxylalkyl group of 2 to 8 carbon atoms as polymerizable monomers.

Alkyl acrylates as component (a) for forming the acrylic elastomerinclude such alkyl acrylates having an alkyl group of 1 to 8 carbonatoms (including alkyl groups containing a substituent group such as acyano group, etc.) as methyl acrylate, ethyl acrylate, n- or iso-propylacrylate, n- or iso-butyl acrylate, n-amyl acrylate, n-hexyl acrylate,2-ethylhexyl acrylate, n-octyl acrylate, 2-cyanoethyl acrylate, etc.,and it is preferable to use ethyl acrylate or n-butyl acrylate.

Alkoxyalkyl acrylates as component (b) include such alkoxyalkylacrylates having an alkoxyalkyl group of 2 to 8 carbon atoms asmethoxymethyl acrylate, ethoxymethyl acrylate, 2-methoxyethyl acrylate,2-ethoxyethyl acrylate, 2-butoxyethyl acrylate, etc., and it ispreferable to use 2-methoxyethyl acrylate or 2-ethoxyethyl acrylate.

Both components (a) and (b) are used together usually in a ratio ofcomponent (a) to component (b) of about 10--about 90 to about 90--about10 by mole.

A portion, that is, up to about 20% by weight, of at least one ofcomponents (a) and (b) can be substituted with other copolymerizablemonomer. Such copolymerizable monomer includes, for example, vinylchloride, vinylidene chloride, acrylonitrile, styrene, vinyl acetate,ethyl vinyl ether, butyl vinyl ether, alkyl methacrylate, alkoxyalkylmethacrylate, etc.

Furthermore, a portion, that is, up to about 10% by weight, of at leastone of components (a) and (b) can be substituted and copolymerized witha dienic monomer. Such dienic monomer includes, for example,divinylbenzene, piperylene, isoprene, pentadiene, vinyl cyclohexene,chloroprene, butadiene, methyl butadiene, cyclopentadiene, methylpentadiene, ethyleneglycol diacrylate, propyleneglycol diacrylate,ethyleneglycol dimethacrylate, propyleneglycol dimethacrylate, etc.

In the present invention, polymerization reaction of at least one ofcomponents (a) and (b) is carried out by using about 0.01 to about 10%by weight, preferably about 0.05 to about 3% by weight on the basis ofthe polymerizable monomers, of an organic peroxide having anon-conjugated double bond as a polymerization initiator.

As the organic peroxide having a non-conjugated double bond, compoundshaving the following formulae (c) and (c') can be used: ##STR1## whereinR₁ and R₂ is H or CH₃ and R₃ is H or an alkyl group of 1 to 12 carbonatoms. The compounds include, for example, t-butylperoxyallyl-carbonate,t-butylperoxymethacrylate, t-butylperoxycrotonate,isopropylperoxyallylcarbonate, etc. ##STR2## wherein R₃ is H or an alkylgroup of 1 to 12 carbon atoms. The compounds include, for example,t-butylperoxydihydrodicyclopentenyl, etc.

It is desirable to use not more than about 5% by weight, preferablyabout 0.001 to about 1% by weight on the basis of the polymerizablemonomers, of an organic mercapto compound or organic sulfide compoundhaving a non-conjugated double bond as a molecular weight-controllingagent, together with the organic peroxide.

As the organic mercapto compound having a non-conjugated double bond,compounds having the following formulae (d) and (d') can be used.

    R.sub.1 CH═CR.sub.2 CH.sub.2 SH                        (d)

wherein R₁ and R₂ are H or CH₃. The compounds include, for example,allylmercaptan. ##STR3##

As the organic sulfide compound having a non-conjugated double bond,compounds having the following formulae (e) and (e') can be used.

    (R.sub.1 CH═CR.sub.2 CH.sub.2 S).sub.2                 (e)

wherein R₁ and R₂ are H or CH₃. The compounds include, for example,allyl disulfide, ##STR4##

Polymerization reaction is carried out in the presence of the organicperoxide, preferably together with the organic mercapto compound by anyof polymerization methods including emulsion polymerization method,suspension polymerization method, solution polymerization method, bulkpolymerization method, etc., preferably by emulsion polymerizationmethod. The polymerization initiator can be used also as a redox system.

The reaction is carried out at a temperature of about -10° to about 100°C., preferably about 2° to about 80° C. in a batch-wise, continuous orintermittent fead system. After the completion of reaction, the formedpolymers are recovered by separation, dependent upon the type ofemployed polymerization method, for example, by adding a coagulatingagent such as acid or polyvalent metal salts, etc. to the reactionmixture in case of emulsion polymerization method or suspensionpolymerization method, and the recovered polymers are then washed anddried, and then subjected to cross-linking with an organic peroxide.

In the cross-linking reaction, about 0.5 to about 10 parts by weight ofsuch an organic peroxide as ordinary organic peroxide, for example,t-butyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 1,3-bis (t-butylperoxyisopropyl)benzene, etc., preferably1,3-bis (t-butylperoxyisopropyl)benzene, is used per 100 parts by weightof acrylic polymer.

In the preparation of a vulcanizable composition, a polyfunctionalcompound such as triallyl isocyanurate, triallyl cyanurate,N,N'-m-phenylenebismaleimide, etc., a lubricant such as stearic acid,etc., various carbon black reinforcing fillers, an antioxidant, etc. canbe added to the composition, if required. When about 0.01 to about 10parts by weight, preferably about 0.05 to about 2 parts by weight, ofthe following various phenols, thiols and sulfides are added thereto per100 parts by weight of acrylic elastomer, the tensile strength can beimproved.

Examples of monophenols:

2,6-di-t-butyl-4-methylphenol

2,6-di-t-butyl-4-ethylphenol

Examples of bisphenols:

2,2'-methylenebis(4-ethyl-6-t-butylphenol)

2,2'-methylenebis(4-methyl-6-t-butylphenol)

4,4'-butylidenebis(3-methyl-6-t-butylphenol)

4,4'-thiobis(3-methyl-6-t-butylphenol)

Examples of polyphenols:

2,5-di-t-butylhydroquinone

2,5-di-t-amylhydroquinone

Examples of thiols:

2-mercaptobenzimidazole and its zinc salt

2-mercaptomethylbenzimidazole and its zinc salt

Examples of sulfides:

dibenzothiazyl disulfide

2-mercaptobenzothiazole and its zinc salt

2-(4'-morpholinodithio)benzothiazole

tetrabutylthiuram disulfide

tetramethylthiuram disulfide

A cross-linkable composition is prepared by adding necessary additivesto a mixture of acrylic elastomers and an organic peroxide cross-linkingagent, and mixing the mixture by kneading in a roll mill or bydispersion in a solvent such as benzene, toluene, xylene, methyl ethylketone, methyl isobutyl ketone, etc., followed by vaporization of thesolvent, and then the thus obtained cross-linkable composition iscross-linked under the ordinary cross-linking conditions.

Vulcanization products using the acrylic elastomers obtained accordingto the present process using an organic peroxide having a non-conjugateddouble bond as a polymerization initiator are better in the scorch time,tensile strength and stretching than the vulcanization products usingthe conventional acrylic copolymers by introducing cross-linkable groupsinto the copolymers and cross-linking the copolymer with an ordinaryorganic peroxide, as shown by comparison of Example 1 (compositionformula A) with Comparative Example 1 and of Example 2 (compositionformula B) with Comparative Examples 2 and 3.

BRIEF DESCRIPTION OF THE DRAWING

Single FIGURE is a graph showing curastometer vulcanization curves inExample 2 and Comparative Example 2.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be described in detail below, referring toExamples and Comparative Examples, where parts are by weight.

EXAMPLE 1

150 parts of water, 0.1 part of sodium sulfate, 5.5 parts of anemulsifier mixture (1.5:2:2) of Emal 10, Emalgen 147 and Levenol WZ(trademarks of products all made by Kao K. K., Japan) and a monomermixture consisting of 99 parts of ethyl acrylate and 1 part ofacrylonitrile were charged into a separable flask provided with athermometer, a stirrer, a nitrogen inlet tube and a pressure-reducingdevice, and after repetitions of degassing and displacement withnitrogen to thoroughly remove the oxygen from the system, apolymerization initiator mixture consisting of 0.16 parts of sodiumhydrosulfite, 0.16 parts of sodium formaldehyde sulfoxylate and 0.16parts of t-butylperoxyallylcarbonate was added thereto to startpolymerization reaction at room temperature. The reaction was continuedfor 6 hours so that the polymerization conversion falled within a rangeof 95 to 98%, and then the reaction mixture was salted out and theprecipitates were thoroughly washed with water and dried, wherebycopolymer elastomers were obtained.

EXAMPLE 2

Polymerization was carried out in the same manner as in Example 1,except that the following monomer mixture was used in place of themonomer mixture of Example 1.

    ______________________________________                                        Ethyl acrylate         42 parts                                               n-Butyl acrylate       27 parts                                               Methoxyethyl acrylate  30 parts                                               Acrylonitrile           1 part                                                ______________________________________                                    

EXAMPLE 3

Polymerization was carried out in the same manner as in Example 2,except that 0.05 parts of vinyl cinnamate was added to the monomermixture of Example 2 and 0.01 part of allyl mercaptan was added to thepolymerization initiator mixture of Example 2.

EXAMPLE 4

Polymerization was carried out in the same manner as in Example 2,except that the same amount of t-butylperoxyallylcrotonate was used inplace of t-butylperoxyallylcarbonate in the polymerization initiatormixture of Example 2.

EXAMPLE 5

Polymerization was carried out in the same manner as in Example 3,except that the same amount of diallyl disulfide was used in place ofallyl mercaptan in the polymerization initiator mixture.

COMPARATIVE EXAMPLE 1

Polymerization was carried out in the same manner as in Example 1,except that the following polymer mixture consisting of:

    ______________________________________                                        Ethyl acrylate   99          parts                                            Acrylonitrile    1           part                                             Allyl methacrylate                                                                             0.05        parts                                            ______________________________________                                    

and the following polymerization initiator mixture consisting of:

    ______________________________________                                        Sodium hydrosulfite      0.01 part                                            Sodium formaldehydesulfoxylate                                                                        0.002 parts                                           t-Butylhydroperoxide    0.005 parts                                           ______________________________________                                    

were used in place of the monomer mixture and the polymerizationinitiator mixture of Example 1, respectively.

COMPARATIVE EXAMPLE 2

Polymerization was carried out in the same manner as in ComparativeExample 1, except that the following monomer mixture consisting of:

    ______________________________________                                        Ethyl acrylate    42         parts                                            n-Butyl acrylate  27         parts                                            Methoxyethyl acrylate                                                                           30         parts                                            Acrylonitrile     1          part                                             Vinyl cinnamate   0.2        parts                                            ______________________________________                                    

was used in place of the monomer mixture of Comparative Example 1.

COMPARATIVE EXAMPLE 3

Polymerization was carried out in the same manner as in ComparativeExample 2, except that the same amount of dihydrodicyclopentenylacrylate was used in place of vinyl cinnamate in the monomer mixture ofComparative Example 2.

Acrylic elastmers obtained in the foregoing Examples and ComparativeExamples were made into various compositions by roll mill kneading, asgiven in Table 1.

                  TABLE 1                                                         ______________________________________                                        Composition (parts by weight)                                                                   A      B      C    D    E                                   ______________________________________                                        Acrylic elastmer  100    100    100  100  100                                 Stearic acid      1      1      1    1    1                                   FEF Carbon black  50     60     50   60   60                                  Substituted diphenylamine anti-                                                                 2      2      2    2    2                                   oxidant (Nogard 445, made by                                                  Uniroyal)                                                                     α, α'-bis(t-butylperoxy-m-                                                          1.5    1.5    1.5  1.5  1.5                                 isopropyl)-benzene                                                            N, N'-m-phenylenebismaleimide                                                                   4.5    4.5    4.5  4.5  4.5                                 2, 2'-methylenebis(4-methyl-6-  0.1  0.1  0.2                                 t-butyl-phenol)                                                               2-mercaptobenzimidazole         0.1  0.1  0.2                                 ______________________________________                                    

These compositions were subjected to press vulcanization at 170° C. for8 minutes and then to oven vulcanization at 175° C. for 4 hours, andnormal physical properties, compression set (150° C., 70 hr, 25%compression) and scorch time (t₅, 121° C.) of the resultingvulcanization products were measured. The results are shown in Table 2together with the type of employed acrylic elastomers and compositionformulae.

                                      TABLE 2                                     __________________________________________________________________________    No.         1  2  3  4  5  6  7  8  9  10 11     12     13                    __________________________________________________________________________    Acrylic elastomer                                                                         Ex. 1                                                                            Ex. 1                                                                            Ex. 2                                                                            Ex. 2                                                                            Ex. 3                                                                            Ex. 3                                                                            Ex. 4                                                                            Ex. 4                                                                            Ex. 5                                                                            Ex. 5                                                                            Comp. Ex. 1                                                                          Comp. Ex.                                                                            Comp. Ex. 3           Composition formulae                                                                      A  C  B  D  B  E  B  D  B  E  A      B      B                     Hardness (JIS-A)                                                                          69 68 66 67 69 69 67 68 69 69 72     71     70                    100% modulus (kg/cm.sup.2)                                                                50 52 43 45 48 50 46 47 49 50 60     58     57                    Tensile strength                                                                          140                                                                              147                                                                              105                                                                              125                                                                              110                                                                              128                                                                              106                                                                              124                                                                              109                                                                              126                                                                              118    95     100                   (kg/cm.sup.2)                                                                 Stretching (%)                                                                            250                                                                              259                                                                              200                                                                              210                                                                              190                                                                              200                                                                              205                                                                              212                                                                              188                                                                              200                                                                              145    149    144                   Compression set (%)                                                                       18 19 16 15 14 13 15 14 14 13 17     15     13                    Scorch time (min.)                                                                        20.0                                                                             23.5                                                                             19.8                                                                             22.0                                                                             18.0                                                                             21.0                                                                             20.5                                                                             22.8                                                                             18.5                                                                             21.2                                                                             8.0    7.8    8.0                   __________________________________________________________________________

Curastometer vulcanization curves were determined for the compositionformula B of Example 2 and Comparative Example 2. The results are shownin Single Figure. In Comparative Example 2, torque was decreased withtime, whereas in Example 2 no such behavior was found at all.

What is claimed is:
 1. A process which comprises polymerizing an alkylacrylate having an alkyl group of 1 to 8 carbon atoms and an alkoxyalkylacrylate having an alkoxylalkyl group of 2 to 8 carbon atoms aspolymerizable monomers with an organic peroxide having a non-conjugateddouble bond as a polymerization initiator, which peroxide is a compoundrepresented by the formula ##STR5## wherein R₃ is a hydrogen atom or analkyl group having 1 to 12 carbon atoms.
 2. A process according to claim1 wherein an organic mercapto or organic sulfide compound having adouble bond is used as a molecular weight-controlling agent togetherwith said organic peroxide as a polymerization initiator.
 3. A processaccording to claim 2 wherein said organic mercapto compound is acompound having the formula

    R.sub.1 CH═CR.sub.2 CH.sub.2 SH

wherein R₁ and R₂ are a hydrogen atom or a methyl group.
 4. A processaccording to claim 2 wherein said organic mercapto compound isdihydrodicyclopentenyl mercaptan.
 5. A process according to claim 2wherein said organic sulfide compound is a compound having the formula

    (R.sub.1 CH═CR.sub.2 CH.sub.2 S).sub.2

wherein R₁ and R₂ are a hydrogen atom or a methyl group.
 6. A processaccording to claim 2 wherein said organic sulfide compound isdihydrodicyclopentenyl disulfide.
 7. A process according to claim 2wherein said organic mercapto or organic sulfide compound is used in anamount of not more than 5% by weight on the basis of the polymerizablemonomers.
 8. A process according to claim 2 wherein said organicmercapto or organic sulfide compound is used in an amount of about 0.001to about 1% by weight on the basis of polymerizable monomers.
 9. Aprocess according to claim 1 wherein the organic peroxide as apolymerization initiator is used in an amount of about 0.01 to about 10%by weight on the basis of the polymerizable monomers.
 10. A processaccording to claim 1 wherein the organic peroxide as a polymerizationinitiator is used in an amount of about 0.05 to about 3% by weight onthe basis of the polymerizable monomers.
 11. A process according toclaim 1 which comprises further subjecting the resulting copolymer tocrosslinking with an organic peroxide.